A retainer for a punch (punch retainer) secures the punch held within it to a die shoe, usually the upper, of a punch press so that the punch may be moved downwards into a die bushing with precision, over and over again so that stringent specifications of a punched sheet may be maintained. The die bushing, in turn, is held in a retainer (die bushing retainer) and secured to an opposed die shoe of the punch press. Typically both the retainers are removably secured to their respective die shoes; and the punch and the die bushing are also removably secured in their respective retainers.
For several decades a "ball lock punch retainer" has been used to secure the punch, and in fewer instances, also the die bushing which is more often clamped to the lower die shoe of the press, or tightly fitted into a recess therein. Despite the many problems associated with the use of a spring-biased retaining ball biased against a helical spring held in an angulated elongated passage within the retainer, this is the industrially favored mechanism because of the relatively low cost of manufacturing its components. However, aside from the relatively poor precision with which the shank (upper portion) of such a punch can be positioned, and the tolerable accuracy with which the point (lower portion) of the punch makes a through-passage ("hole" for brevity) of arbitrary cross-section in a sheet of stock being punched, a serious problem is that it is routinely an arduous and frustrating task to release a punch when it is to be replaced. One of the reasons is that repeated operation of the punch distorts the shape of the ball, which then becomes immovably lodged against the punch or against a helical spring against which the ball is biased. The problem of replacing the punch is worse when the ball is sheared, which typically happens when the stripping force exceeds that which the ball can withstand. In operation, punches are routinely subjected to unexpectedly large stripping forces typically cause by galling of the point.
An inherent result of using a ball seat or pocket in the shank of a punch to lock it with a ball is that, the shank of the punch is of necessity, cylindrical. If the point of the punch is non-circular in lateral cross-section, it can be sharpened only until the point is used up and the shank is reached. Moreover, by reason of the clearances required between the pocket and the ball, and the relatively small force exerted by the spring against the ball, it is difficult to maintain concentricity with tolerance less than 0.001 inch. Particularly when the shape of the hole to be punched is other than circular, the shank is not held tightly and non-rotatably in its elongated passage with the result that the play between the ball and the pocket results in slight but unacceptable variations in orientation of the hole punched. These problems are more readily envisioned by reference to FIGS. 1 and 2 in which the prior art mechanism is briefly described. Moreover, the structural differences and their effect on the forces exerted on a tool to be replaced, when compared to those of the present invention, will more readily be appreciated.
Similar considerations apply to securing a forming tool which operates in a forming press and which forming tool is typically secured in a manner analogous to a punch. A commonly used forming punch has a point for making the desired hole in a sheet of stock, and has an upwardly flared conical portion directly above the tip of the point. The flared portion serves to provide desired concavity. Hereafter, for brevity and convenience, a punch and a forming tool or forming punch, and a die bushing are together referred to by the term "tool", and are identified individually when specifically referred to.
Referring to FIGS. 1 and 2, there is illustrated a retainer block indicated generally by reference numeral 10 and a conventional punch 20 held therein. A forming toot if used, would be analogously held. The retainer block 10 includes a through-hardened backing plate 12 conforming to the upper surface of the retainer block, both being adapted to be secured to an upper die shoe of a punch press or other machine with a punching or forming function by suitable fastening means such as Allen-head screws (not shown). Since a tool (punch or forming) is generally used in a vertical attitude in a punch or forming press, the description herein refers to upper and lower in relation to such attitude. The retainer block 10 is provided with a cylindrical bore or tool socket 14 in which is slidably inserted and removably secured the shank (upper portion) 22 of the punch 20, the lower portion of which is an oval-shaped point 24. Block 10 is also provided with a cylindrical bore 15 which is angularly disposed relative to the bore 14 and which extends inwardly and downwardly into the retainer block 10 so as to partially intersect socket 14. The partial intersection occurs because the lower end of the bore 15 is provided with a stepped surface forming ball seat 13.
A retainer ball 16 is movably disposed in bore 15, and a helical compression spring 18 is snugly held in the bore 15 with one end abutting the backing plate 12 so as to urge the ball 16 outwardly of the intersecting portion of bore 15. Though the ball projects into the socket 14 the ball cannot escape (into the socket 14). The retainer block is also provided with a through-passage or release-bole 17 through which a thin rod or drift pin is inserted to push the ball upward and move it out of the ball seat 13 when the punch 20 is to be removed. To replace the ball 16 when it gets distorted or damaged, the retainer block 10 is removed from the backing plate 12 and the spring and ball removed through the top of bore 15.
The shank 22 is provided with a semi-pocket or ball seat 25 shaped generally like a one-half of a falling tear drop viewed in longitudinal elevation, and which is adapted to receive locking ball 16 to releasably lock the punch 20 in the bore 14. The pocket's upper portion 26 appears as a straight section forming a continuation of the bore 15; and the lower portion is provided with a return section 28 which is curved upon a radius greater than the radius of the ball 16 so as to connect the deepest part of the pocket 25 to the surface of the shank. When the ball 16 is held in pocket 25 its bottom may be in contact with the ball if the radius of section 28 is substantially greater than that of the ball; or, if the radius of the ball is substantially greater than that of the return section 28, the extreme edges 34, 35 of the pocket 25 will contact the ball.
To appreciate the advantage of locking a punch precisely positioned in the retainer block, the problem with using a pocket and retaining ball is illustratively presented in FIGS. 3 and 4 so it may be more readily visualized. Both problems, namely of securing the tool to the die shoe, and positioning the punch (and die bushing) precisely, is particularly severe with relatively small diameter punches having a shank less than about 7.6 cm (3 ins.) in diameter. A larger diameter shank may be secured and precisely positioned with screws and dowels through the shank and die shoe. In FIG. 3 is shown a shank 22A having a pocket 25A with an arcuate section having a radius substantially greater than that of ball 16A, allowing the punch to rotate slightly in either direction, as shown by the arcuate double-headed arrow, so that accurate alignment between a non-circular punch and its corresponding die bushing cannot be maintained. In FIG. 4, in shank 22B, the arcuate section of pocket 25B has a radius smaller than that of ball 16B so that it engages the corner portions 34B, 35B of the pocket in the shank. Under operating conditions which generate high forces, depending upon the relative hardness of the ball and the shank, either one or the other, or both are distorted or damaged; at the very least the extreme edges 3435 of the pocket are pushed outward as shown at 38, 39.
Thus for optimum locking it is desirable to have the diameter of the ball accurately adapted to fit in the pocket so as to have the pocket contact the ball at two opposed points 33 inwardly spaced apart from the edges 34, 35 as shown in FIG. 2, the distance inward being chosen so as to avoid forcing the extreme edges 34, 35 outwards. Such precision is difficult to achieve in practice, and is proportionately so expensive as to be uneconomical. When achieved it will be evident that, the ball being a sphere, the contact at 33 is essentially point-contact with the surface of the pocket 25 and not substantially different from the point contact between the ball 16B and shank 22B with the pocket 25B.
To avoid using a ball lock mechanism, wedges have been used to lock a punch transversely in a retainer as illustrated in U.S. Pat. No. 3,137,193, the shank is provided with a flat (shank flat) on one side thereof which flat engages a cooperating flat formed on a tapered retaining pin fitting within a transversely extending opening formed in the punch retainer. Since the tapered pin cannot prevent the punch from moving vertically the shank must also be held by a pin the inner end of which has a sloping wedge surface which is adapted to engage a cooperating wedge surface formed on the shank of the punch as a part of a cutout on the opposite side from the shank flat. Even if one accorded this means for holding a punch in a retainer great merit for accuracy, it is evident that such a punch and retainer function to wedge the shank by contact with two inclined surfaces, each of which serve to wedge the shank laterally, not vertically. The inclined surfaces form acute angles with the horizontal in a horizontal plane, that is, "laterally acute"; not with the vertical in a vertical plane, that is "vertically acute". Moreover, such a mechanism is complicated and expensive to produce. Equally evident is why the ball lock punch retainer is the current standard for the machine tool industry.
In an analogous manner, when it is inconvenient or impractical to clamp a die bushing in a die-receiving hole, or one seeks either to avoid press-fitting a die bushing in the die-receiving hole, or using a ball lock mechanism to do so, the die bushing may be held as shown in U.S. Pat. No. 3,535,967 to Whistler et al. The die bushing is accurately positioned in a flexible retainer into which it is press-fitted and is held in the die retainer block by providing one side of the bushing with a flat surface, the flat cooperating with a corresponding flat on an aligning pin disposed transversely within a transversely extending opening in the die retainer.
The goal of this invention is to provide a locking means for a tool in a retainer block, which locking means will accomplish what the ball lock does, and much more, not only with respect to precision and strength, but also for economy and ease of operation; and to permit quick replacement of the tool by releasing it in its tool-receiving cavity with a force which is proportional to the pitch of threads in the screw means which secures the wedge in its wedge cavity to the backing plate of the retainer block.